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Rain Garden Design and Construction
WHAT IS A RAIN GARDEN? Rain gardens are landscape features, usually emphasizing native plantings, that are designed to capture storm water runoff and allow it to filter into the soil, and potentially groundwater, below. The simplest form of rain garden is a shallow depression located downslope from impervious surfaces. Other names for this type of feature are bioswale, bioinfiltration pond, and bioretention area. Rain gardens can be constructed in a variety of locations including residential and commercial landscapes, municipal parks, along roadways, in parking lots, and even in agricultural settings. Although rain gardens are suited for just about any climate that receives rain, the materials presented here focus on residential applications in southern California. For information related to other climate zones, references are available below. RAIN GARDENS AND SUSTAINABILITY Rain gardens mimic natural landscape phenomena that capture rainwater, thus restoring native hydrology to areas that may have been degraded through development or neglect. As man-made features, their popularity has increased in the last decade with the emergence of Low Impact Development (LID) planning and best management practices. More recently, the Sustainable Sites Initiative (SITES) has developed a rating system specifically aimed at promoting sustainable land development and management practices for sites with and without buildings. Part of the impetus for this movement has come out of the recognition that human habitation has created an abundance of impervious surfaces, especially in urban environments. These impervious surfaces – from rooftops and driveways to sidewalks, roads, and parking lots – prevent rain from soaking through to the soil beneath them. Instead, the pollutants that collect on these surfaces enter storm water runoff and end up in downstream bodies of water. The following graphic from the Environmental Protection Agency details the effects of increasing amounts of impervious pavement. As the percentage increases, less water is available for infiltration or evapotranspiration. According to the Environmental Protection Agency, these nonpoint source contaminants comprise a major percentage of the pollution in our surface waters. Incredibly, impervious surfaces in urban areas generate more than five times the runoff of a woodland area of the same size. In addition to carrying pollutants to streams, lakes, and coastal waters, storm sewer systems concentrate runoff volumes in smooth conduits that allow the water to gain speed and erosional power before it empties into bodies of water. This can damage streambeds, vegetation and important habitat. The heat that is generated by impervious surfaces travels with storm water and can have important adverse affects on aquatic fauna as well. Rain gardens are one strategy – others include green roofs, disconnecting downspouts, installing pervious pavement, using graywater and harvested rainwater for landscapes – that promotes sustainable water management. Specifically, rain gardens: * Help recharge groundwater resources * Filter pollutants * Reduce storm water runoff volume * Create habitat * Reduce demand for potable water for landscapes (and indirectly, the energy use associated with centralized water provision) * Help increase evapotranspiration (through evaporation and via plants) * With careful plant selection, they can also provide great beauty right where you live. DESIGN CONSIDERATIONS of the information for this section was drawn on the publication Raingardens: A How-to Manual by the Wisconsin Department of Natural Resources (link available below) In designing a rain garden, several factors should be taken into consideration. First, an appropriate location needs to be found down slope from the surfaces that will drain into it. Second, the size of the rain garden needs to be calculated based on water quantities, slope, and soil conditions. Finally, proper plant selection is important for maximizing the cleansing and infiltrating qualities of the garden as well as for aesthetics and maintenance. Location All rain gardens should be located a minimum of 10 feet from a raised foundation and 5 feet from a slab foundation. This is to ensure that water does not back up under structures and damage them. Choose a location that takes advantage of natural drainage patterns (and gravity) and is relatively flat. The closer the garden is to a structure, the more it will be fed solely by rooftop runoff via downspouts. The more distant it is, the more opportunities there are to capture runoff from other surfaces (driveways, sidewalks, patios) and any lawn surrounding the home. Most rain gardens thrive best in full to partial sun locations, although shady locations can also succeed. Take care to avoid areas directly under trees as they may complicate maintenance. Also, carefully consider how the rain garden will integrate into the current landscape and note views from both inside the home and from outdoor vantage points. Rain gardens located next to patios can provide fragrance and color to outdoor gatherings. Alternatively, they can help provide partial screening from the sidewalk or neighbors’ properties. Calculating size The size of a garden will depend on how much water will be captured, the slope of the land, and what kind of soil is present. Drainage areas For rain gardens that will capture primarily rooftop water, first calculate the surface area of your roof. This can be done based on the flat square footage of the first floor and rounding up. Then determine how much of the roof is drained to the downspout(s) that will feed the rain garden. If it is 25% for instance, multiply the roof area by 0.25. This will give you the roof drainage area. Example: A house with 2475 sq. ft. on the ground level (roughly 45ft x 55ft) has five downspouts. Two of them that drain about half the roof will feed the rain garden. Multiply the roof area (rounded up) 2500 x 0.5 = 1250 sq. ft. of roof drainage area For rain gardens farther from structures (30 ft or more), consider the surface area of hardscapes (drives, sidewalks, etc.) and lawns up hill from the garden location as well. It is possible that only a portion of these areas will drain to the garden. Determine the dimensions of the areas that will drain, multiply length by width to determine square feet and add this figure to the roof drainage area. This is the total drainage area. Slope of land The depth of a rain garden will depend on the slope of the lawn it is placed in. On fairly level land, a simple depression can be created according to drainage area and soil type. For sloped land, the angle of slope will also affect garden dimensions. To determine slope, pound a stake at the upper and lower edges of the garden area – roughly 15 feet apart. Tie a string between the stakes and level it, making sure the string is at ground level on the upper stake. Next measure the length of the string and the height on the downhill stake between the string and the ground, both in inches. Divide the height by the width and multiply by 100. This will give you a % slope. Example: The distance between the two stakes is 180 inches. The height of the area on the downhill stake between the ground and the level string is 10 inches. Slope equals 10 ÷ 180 x 100 = 5.5% Because a berm will be constructed on the downhill side of the garden to help detain water, the following guidelines can be used to determine overall depth of the basin. If the slope is: *Less than 4%, the garden should be 3-5 inches deep. *Between 5% and 7%, the garden should be 6-7 inches deep. *Between 8% and 12%, the garden will need to be about 8 inches deep. For slopes greater than 12%, you may want to contact a professional landscaper. Soil type Next, the soil type where the garden will be constructed must be considered. Because less dense soils such as sand and loam drain more quickly, the area of a rain garden with these soils can be smaller than ones built in clay-like soil. There are several ways to determine what type of soil is present. Perhaps the simplest method is to hold some moist soil in your hand. When squeezed: *Clay will hold together well and feel sticky *Loam soil will hold some shape and feel spongy. The mixture of components and organic matter usually give it a dark brown to black color. *Sandy soil will not hold shape and easily slips through the fingers. Another method is to test the infiltration rate of the soil. To do this, dig a hole 1 foot deep and 6 inches in diameter. Fill it three times, allowing the water to soak in between fillings. Fill the hole a fourth time and observe how long it takes the water to soak into the saturated soil. Sandy soils will drain at approximately 1-2 inches per hour. If there is water remaining after 12 hours, you can assume the soil has significant amounts of clay and will need to be amended for the finished rain garden. *'It is important to note that water in rain gardens should not be allowed to stand for more than a maximum of 48 hours in order to avoid creating habitat for mosquitos and other vectors.' With knowledge of the drainage area, soil type, and garden depth, the size of the rain garden can be calculated. Take the drainage area figure and multiply by the appropriate rain garden size factor as found in the table below: From the garden example used above, we know: *Total drainage area = 1250 sq. ft. *Slope = 5.5% which suggests a garden depth of 6 inches If we assume there is silty (loam) soil, the calculations would be as follows: 1250 x 0.25 = 312.5 sq. ft. A similar rain garden located farther from the home would need to be significantly smaller (1250 x 0.06 = 75 sq. ft) due to the additional absorptive properties of the lawn/plantings above the garden. Choosing Plants Both technical and aesthetic factors should be considered in selecting plants for a rain garden. Native plants are well suited for rain gardens as they are naturally adapted to periods of rain and drought, depending on your climate. With their deep root structures, they facilitate water infiltration, help prevent erosion, and provide organic material to the soil with their cyclical growth and die-off. Although it is tempting to think of rain gardens as wet environments, in reality they are dry gardens that are periodically flooded. This is an important concept when selecting plants. More information on this approach can be found here. Natives present a wide variety of options in terms of flowering plants, grasses, small bushes and even trees. When choosing specific plants for a plan, consider including ones of different heights, textures, bloom time, and color. This will extend the season of the garden and make it a more attractive addition to your landscape. Plants can also be selected for fragrance and their ability to attract wildlife (butterflies, hummingbirds, etc.). Grouping species in clumps of 3-7 plants gives a naturalistic feel to the design and emphasizes the qualities of each species. More formal layouts can be achieved with edging and planting in straight lines. Using stones, decorative benches and other accents can enhance the overall cohesiveness of the garden. Sample rain garden plans and plant lists are widely available. Here is a selection from sources in Southern California and other locations. For Santa Barbara: *The City of Santa Barbara’s Best Management Practices (BMP) Post Construction Technical Guidance provides plant lists for this area. (Appendix G) *Surfrider Foundation has published Ocean Friendly Gardens – a guide to landscapes that can help reduce polluted runoff. For Elsewhere (note, the native species will be different but basic design concepts can be transferred to any environment): *The Wisconsin Department of Natural Resources has published “Rain Gardens: A How-to for Homeowners” with detailed information on planning a garden including specific layouts suitable for the Midwest. (DNR Publication PUB-WT-776 2003) Available at: http://www.dnr.state.wi.us/) *Sample rain garden designs for shady or sunny locations by Candace Stoughton *Raingardens.org has also compiled a list of resources for garden designs. INSTALLATION Step by Step Guide Creating the garden basin After identifying the location and size of the rain garden you will construct, the first step will be to layout the outline and start digging. A garden hose or playing field chalk work well for defining the outline. Depending on the size of the garden, the digging can be accomplished by hand or with the aid of rented equipment. Always check for underground utility locations before getting started. Turf will likely need to be removed first. The process can be aided by killing the grass beforehand, either with herbicide or, preferably, by covering the grass for several days with plastic. Remove the dead grass and begin shaping the garden. As you go be sure to check the depth and levelness frequently. Soil removed from the depression can be collected on a tarp or near the downhill edge of the rain garden, where it will be incorporated into the berm. Berm construction Once the basin is at the desired depth and level, mound up soil on the downhill slope to an elevation level with the uphill side (where water enters). The berm should be rounded in shape and continue around the side of the garden enough to contain the desired runoff. Be sure the sides of the berm are gently sloped for best performance. Next, tamp down the soil firmly. Jump on it if you like. To decrease erosion, landscape cloth or biodegradable jute netting (a more ecological choice) can be draped over the surface and anchored under additional dirt or with large rocks at the edges. Grasses do particularly well in stabilizing berms with their root structures, although other perennials will also work. Amending soil The more porous the soil, the better it is able to infiltrate water. If your soil is clay-like, you may want to amend it before you put it back in the basin. This can be achieved by adding sand and compost to the clay. An ideal ratio might be: *50-60% sand *20-30% compost *20-30% topsoil When filling the basin with soil, be sure to keep the top surface below the level of berm. Also, avoid compacting it too much. This is the number one cause of rain garden failure as plants are unable to grow the appropriate root structure. Planting The final step is putting the selected plants into the ground in the desired layout. This is where creativity can really come into play and the results become very visible. Once the plants are all in, water the garden regularly (twice a week) until plants are established. Adding mulch on top of the soil will help keep it moist, especially during hot weather. Visualizing the process Many videos of rain garden installations are available online. Here is a sampling: A short video illustrating installation. A longer video from Metro Blooms CHALLENGES Although rain gardens are relatively easy projects to undertake that make significant contributions to sustainable water management, there are also some challenges associated with them. Potentially unsustainable aspects In areas with high presence of fertilizers and heavy metals, there is some concern that these contaminants could be concentrated in the garden soils rendering them less healthy than before. Certain plant choices (mustard, alpine pennycress, and pigweed) can mitigate these dangers as they take up the metals. This strategy – called phytoremediation – has been used successfully on contaminated industrial sites. Its advantages are that it is inexpensive and the least harmful of remediation strategies. Unfortunately, it also has limitations including: its effectiveness is limited to the root depth of the plants; groundwater contamination may still occur; in highly toxic areas plants may die; in agricultural contexts, these heavy metals may enter the food chain. Another challenge is in managing the growth of unwanted vectors such as insects and bacteria. In the case of mosquitoes, standing water is required for 7 days to enable larva incubation. If a rain garden is designed properly, this will be avoided. In the case of bacteria, the primary hazard source would likely be a greywater system feeding the garden. Care needs to be taken to properly plumb and filter these water sources. Contamination can also come from animal droppings, so picking up after pets is encouraged. Finally, in order to protect groundwater tables, it is important to ensure that rain gardens are dug only to the required depths and do not directly contact groundwater basins. This is more difficult in commercial settings or where the area of a garden or detention area is quite large. For residential applications, this risk is minimal. In any setting where the bedrock is shallow, rain gardens should be avoided as they will be ineffectual. Limitations for Widespread Adoption One of the primary complaints about rain gardens is that they are ugly. While the sentiment is understandable, this is a limitation of implementation or expectations more than a quality intrinsic to rain gardens themselves. With proper design and maintenance, rain gardens can add substantial aesthetic value to the neighborhood in which they are built. Educating one’s neighbors about the substantial benefits of rain gardens can also help with their acceptance. See "Additional Resources" below for more information. A second and perhaps more serious barrier to widespread adoption of rain gardens is the issue of scale. In residential areas, typically individual homeowners are responsible for undertaking design and construction of a rain garden. While costs can be kept low (roughly $3-5 per sq. ft) by doing it oneself, this requires an investment of substantial time. As not all homeowners will have the time (or inclination for that matter) to build a rain garden, implementation will likely be uneven across cities. The Future While rain gardens are decidedly low-tech, advancement in design and technical understanding occur on a continuous basis. For instance, in highly toxic areas, some designers have begun adding "flush strips" at the input edges of the rain garden. These are constructed primarily of gravel or decorative aggregate and are intended to capture the most concentrated flows of pollutants following a rain storm. They may also have an added benefit of helping capture the heat associated with these flows, thus moderating the effects on plants in the interior of the garden. Several studies have been done to assess the impact of rain gardens on collection and infiltration rates of specific metals such as zinc, lead, and copper, as well as organic compounds such as nitrate-nitrogen and phosphorus. In one review of the findings, Michael Dietz, of Utah State University, concluded that while rain gardens are successful in capturing the majority (95%) of heavy metals in storm water runoff, they are less successful with nitrate-nitrogen (24%), and even less so with phosphorous (net contribution). These results call for more focus on how mitigate these effects and may lead to modifications in ideal soils, design, or plant selections. As water resources become more scarce, the trend toward conservation will certainly gain momentum. It would be ideal if installing a rain garden in every residential landscape were achieved. Toward that end, local governments and non-profit organizations can provide incentives to encourage rain garden construction as part of city-wide water management strategies. In the Santa Barbara area, the SMART Landscape Rebate Program is offering rebates on materials for water-friendly projects. Many other resources are available at the state and federal levels - a selection of which can be found below. ADDITIONAL RESOURCES General: The U.S. Environmental Protection Agency has a multitude of resources, including materials for homeowners, kids, construction sites, and for public meeting handouts. *The EPA's handbook on Low Impact Development The National Resources Defense Center has good introductory information on stormwater strategies and several case studies for review. The Low Impact Development Center hashas guides, case studies, and information about their projects - such as Green Streets Low Impact Development Urban Design Tools Website offers information on bioretention and other LID strategies. Sustainable Sites Initiative (SITES) Wisconsin's Department of Natural Resources - Raingardens: A How-To Manual for Homeowners. Available at: http://www.dnr.state.wi.us/ West Michigan Environmental Action Council maintains Raingardens.org with a step-by-step guide to design and installation. Specifically for the Santa Barbara area: The City of Santa Barbara has created two useful documents: *Storm Water Best Management Practices Guidance Manual *A Homeowner's Guide to Managing Storm Water Santa Barbara Channelkeeper has how-to summaries of several LID strategies The Santa Barbara Permaculture Network promotes hands-on learning through co-operative projects in the community. They regularly build bioswales, rain gardens, and rain harvesting systems with volunteered labor. Category:Challenges